1. Field of the Invention
This invention relates generally to monitoring apparatus for identifying instabilities in machinery. In particular, the monitoring apparatus of the invention identifies multi-frequency vibrations of a machine which are above a predetermined magnitude and determines if a vibration of a predetermined frequency is present so as to generate a signal to modify operation of the machine. Still more particularly, the monitoring apparatus of the invention is provided for testing of a rocket thruster.
2. Description of the Prior Art
This invention resulted from an acceptance testing project for space shuttle reaction control system (RCS) thrusters. Instabilities in rocket thrusters have been extremely rare, only occurring in approximately three out of 50,000 firings. Even though rare, it is important to detect such instabilities of an RCS thruster because of its critical importance to a space flight mission, especially one like manned shuttle flights.
Previous methods of detecting instabilities of an RCS thruster during acceptance testing was to fit the thruster with an accelerometer to produce an analog signal representative of the multiple frequency vibrations of the thruster when it was fired. A broad frequency spectrum was obtained of the vibrations signal to determine components of high magnitude or "g" loads. But high g loads may be caused by any one of a number of factors other than thruster instability. For example the test stand on which the thruster is placed may vibrate.
Consequently, when high g loads were detected, engineers were required to stop the test and analyze the test data to determine if the high g loads were occurring at the "critical" frequencies indicative of rocket instability or at some other frequency due to other causes. The critical frequencies which indicate rocket instability are determined by the geometry of the rocket engine cavity. Such critical frequencies are also known as one or more resonant frequencies of the thruster engine.